1. Field of the Invention
The present invention relates to electric arc welding, and more particularly it relates to methods of electric-arc condenser-discharge percussion stud welding, which are also called the Vang welding methods.
The disclosed method of electric-arc condenser-discharge percussion stud welding can be employed for production of welded structures in general engineering, ship-building, instrument-making and civil engineering.
2. Description of the Prior Art
There is widely known a method of electric-arc condenser-discharge percussion welding of studs to metal sheets, plate and other articles ("Avtomaticheskaya svarka" Magazine, No. 9, 1973, "Naukova Dumka /Kiev/, N.A. Chvertko, "Udarnaya kondensatornaya svarka shpilek i shtiftov", pp. 51-53), according to which, in order to ensure reliable ignition of an electric arc between the end of a stud and the article, a projection is formed on the end of the stud by cold upsetting, the length and diameter of this projection being determined by the main diameter of the stud and material of the parts to be welded. The projection on the end of the stud to be welded is shaped as a wedge, as a cone or otherwise. A spring mechanism is operated to set the projection of the stud upon the surface of the article. A bank of welding condensers (capacitors) precharged to a required voltage value is electrically connected to the stud and to the article, so that the welding current starts flowing through the projection of the stud and the article. When the value of the discharge current of the condenser bank reaches its peak, the projection becomes explosion-like evaporated by the high current density (to 10.sup.6 A/cm.sup.2) in the projection. Owing to the high temperature of metal vapors, the gap across the stud and the article is ionized, and an electric arc is ignited, the arcing taking place in the vapors of the stud and article being welded. The length of the electric arc and its arcing time depend on the length and diameter of the projection of the stud. The end of the stud and the surface of the article underlying it are fused. The action of the spring mechanism brings the end of the stud closer to the article, so that the length of the electric arc gradually diminishes, and when the end of the stud is introduced into the molten metal of the article, the electric arc is extinguished. When the molten metal completely hardens, the welding cycle is completed.
The formation of a projection on the end of a stud is a relatively complicated operation, as comparatively high precision of the length and diameter of the projection is essential for proper repetition of the operation of making quality welded joints.
There is also widely known a method of automatic arc welding of a curvilinear butt joint of a variable width in a V-groove with a consumable electrode (SU, A, 941052). The method is that of gas-shielded d.c. welding. First, an electric arc is ignited between the electrode and the welded structure. With the arc ignited, the electrode is moved along the weld, while being simultaneously oscillated across the weld. In the course of the welding the voltage of the welding arc is measured red, and when the value measured decreases by 10-20% from the predetermined value and this variation lasts for 0.025-0.1 s, owing to the variation of the length of the arc gap as the electrode approaches the edge of the structure being welded, the motion of the electrode is reversed. As a result, the electrode is guided along the weld with sufficient accuracy, and the variable-width groove of the welded structure is uniformly filled with metal across its width. As each successive bead is applied (the welding operation being conducted in several passes), the rate of the welding is automatically varied.
The reversing of the motion of the electrode following the varying voltage of the electric arc is feasible only when the voltage varies within a 10% to 20% range, because if the voltage of the electric arc decreases by a greater value, the welding duty is adversely affected. The 0.025-0.1 s lag in sending the command for reversing the motion of the electrode slows down the process.
There is further widely known a method of electric-arc condenser-discharge percussion stud welding (Thyssen Technische Berichte, No. 2, 1982, A. Frings et al., "Lichtbogenbolzenschweissen and Kaltgewalzten Stahlfeinblechen", pp. 161-170), including prepositioning a stud on an article, retracting the stud to be welded from the article while igniting an auxiliary electric arc between them, and moving the stud towards the article by the spring mechanism of the welding head. After the predetermined time of arcing of the auxiliary arc, a discharge of a capacitor or condenser bank is initiated, so that the welding current flows through the stud to be welded and the article, fusing the end surface of the stud and the underlying surface of the article. Upon the preset time of arcing of the welding electric arc the stud is introduced into the molten metal of the article. Then the stud to be welded and the article are held together or butted to form a welded joint. In this process, the setting of the stud to the topmost, retracted position requires the use of special-design gauges of high precision, as the timing of the discharge of the condenser bank is determined by the time of the motion of the stud relative to the article.
However, the process would not ensure adequate repetition of the operation of butting the stud and the article, caused by insufficiently stable conditions of the performance of the spring mechanism of the welding head. A varying diameter of studs would vary the mass of the spring mechanism, requiring an adjustment of the resilience of the spring mechanism of the welding head.
This object is attained in a method of electric-arc condenser-discharge percussion stud welding, including prepositioning a stud on an article, retracting the stud from the article while igniting an auxiliary arc between the end of the stud and the article, moving the stud towards the article, discharging a condenser across the end of the stud and the article, introducing the stud into the molten metal of the article, and butting the end of the stud and the article to form a weld, in which method, in accordance with the present invention, while moving the stud towards the article, the voltage drop across the auxiliary electric arc is measured, and from the value thus measured is subtracted the sum of the electrode-adjoining voltage drops at the end of the stud and at the article, and the discharge of the condenser is initiated when the voltage drop value thus obtained varies by 10% to 90% from the initially obtained voltage drop value.
The herein disclosed method of electric-arc condenser-discharge percussion stud welding provides for enhancing the efficiency factor of the welding operation by prolonging the arcing time of the welding electric arc, which permits to extend the range of weldable studs.